Field of the Invention
Embodiment of the present invention relate to a switching power supply circuit which prevents a boost diode in a chopper circuit from breakdown due to overheating even in an event of short-circuit in a bypass diode that bypasses the circuit between the input side terminal and the output side terminal of the chopper circuit.
Discussion of the Background
A critical mode power factor correction (PFC) type switching power supply circuit is known as a DC power supply device for use in a power supply section of a variety of electronic devices. FIG. 4 shows a schematic construction of this type of switching power supply circuit 1. This switching power supply circuit 1 has a construction of a chopper circuit of a boost type comprising an inductance L receiving a DC voltage Vin, and a semiconductor switching element (also referred to simply as a switching element) SW for controlling the current running through the inductance L.
The switching element SW is ON/OFF-controlled by a switching control section Scont, which is a power supply IC. An AC voltage Vac is full wave rectified by a diode bridge circuit DB and then smoothed with an input capacitor Cin to obtain the DC voltage Vin for supplying to the inductance L. A DC voltage Vout is obtained from the inductance L through a boost diode D1 and stored in an output capacitor Cout, and then delivered to a load (not shown in the figure).
Voltage dividing resistors Rv1 and Rv2 in FIG. 4 detect the output voltage Vout, which is a voltage across the output capacitor Cout, of the switching power supply circuit 1, and feed-back a detected feedback voltage to the switching control section Scont. A shunt resistor Ris detects the output current Is of the switching power supply circuit 1. A resistor Rrt adjusts the gradient of a ramp wave signal for PWM control, which will be described below. The resistance values of the resistors Rv1, Rv2, Ris, and Rrt are determined corresponding to specifications of the output power of the switching power supply circuit 1.
The switching control section Scont ON/OFF-controls the switching element SW using a pulse signal P corresponding to the output power, or output voltage Vout, to the load. Upon turning ON of the switching element SW, the current through the inductance L rises from zero. Upon turning OFF of the switching element SW, the current through the inductance L falls to zero. Thus, the switching control section Scont works as a chopper.
The chopper circuit has a bypass diode D2 between the input terminal and the output terminal thereof. The bypass diode D2 allows the DC voltage Vin onto the inductance L to bypass the inductance L and the boost diode D1, and applies the DC voltage Vin to the output capacitor Cout. The bypass diode D2 avoids overheat breakdown caused by overcurrent through the boost diode D1 in startup period of the chopper circuit.
In the startup period of the chopper circuit, the output capacitor Cout has no stored charges, and the output voltage Vout is at zero volts. If an AC input voltage is applied to the chopper circuit without the bypass diode D2, a large charging current flows from the inductance L through the boost diode D1 to the output capacitor Cout. This charging current would cause overheat breakdown of the boost diode D1. When the bypass diode D2 is provided, however, the charging current to the output capacitor Cout flows through the bypass diode D2. Thus, the boost diode D1 is protected against overheat breakdown due to the charging current.
The bypass diode D2 used has a greater rated current carrying capacity than the boost diode D1 so that the boost diode D1 does not break down due to the charging current. The bypass diode D2 is used only at the start up time of the chopper circuit, and not used in the ON/OFF-control period of the switching element SW. Thus, the bypass diode D2 can exhibit a long reverse recovery time.
Thus, in operation of the chopper circuit, the boost diode D1, which exhibits a short reverse recovery time, works in the ON/OFF-control period of the switching element SW, and the bypass diode D2, which exhibits a large rated current carrying capacity, works at the start up time of the chopper circuit. Therefore, a switching power supply circuit 1 that exhibits both high reliability and high efficiency can be constructed by providing the bypass diode D2. A switching power supply 1 having such a construction is disclosed in detail in United States Patent Application Publication No. 2006/0033480 and United States Patent Application Publication No. 2008/0316779, for example.
When the bypass diode D2 is short circuited, an electric current flows, as shown in FIG. 5A, from the inductance L through the switching element SW in the ON period of the switching element SW. In the OFF period of the switching element SW, as shown in FIG. 5B, the energy stored in the inductance L flows through the bypass diode D2.
Thus, in turning OFF time of the switching element SW, the current through the current detection resistor Ris, which is connected to the switching element SW in series, is rapidly interrupted. This results in earlier timing of zero current detection through the resistor Ris for performing the control to reduce energy loss in the switching element SW. This in turn results higher switching frequency for the switching element SW.
In this time, the output voltage Vout across the capacitor Cout is suppressed low regulated with the input voltage Vin through the short-circuited bypass diode D2. Thus, the switching control section Scont executes PWM control to expand the ON width of the pulse signal P in order to increase power delivery to the output capacitor Cout. This increases the current flowing in the switching element SW, and makes the overcurrent detection function for the switching element SW into action to turn OFF the switching element SW.
Consequently, when the bypass diode D2 is short-circuited, the switching element SW continuously operates as shown in FIG. 6B at a higher oscillation frequency than in the normal operation period sown in FIG. 6A. This continuous operation at a higher oscillation frequency than in the normal operation period is conducted at the maximum ON width under the limited ON width of the switching element SW due to overcurrent detection.
In this switching operation of the switching element SW, a DC current flowing through the bypass diode D2 is superimposed on the current delivered to the output capacitor Cout through the boost diode D1. This increases the current flowing through the boost diode D1 and may cause abnormal heating of the boost diode D1. To cope with this abnormal heating of the boost diode D1, it could be considered to use a plurality of parallel connected diodes or to use a diode with a higher rated current carrying capacity than normally required current carrying capacity. These measures, however, are costly.